of observed states of the network, that is normally tightly coupled with corresponding relationships in physical space. Thus, under constant conditions, an external observer can reconstruct a rat's position from the collective firing pattern (e.g., Wilson and McNaughton, 1993;Zhang et al., 1998). The expression, storage, and retrieval of these neuronal 'maps'provides an excellent forum in which to explore the extent to which age-related alterations in neural system level dynamics can be linked at lower levels to changes in cellular physiology, and at higher levels to changes in the behavioral expression of memory 16